I think the most common phrase (though not exactly the same) might be salpimentar a gusto, though it does really mean that you add as much or as little salt and pepper as you like.
If you are referring to a last minute adjustment, perhaps we would say comprobar/ajustar/corregir el punto de sal. This is another common expression, but of course only mentions one type of seasoning.
My suggestions are only what could be considered everyday phrases. I'm sure someone else will offer more exact ones if you need to be more accurate.
(Juandiego and I were simultaneous in our answers)